Customised Dewatering Solutions
Premature Flocculating Powder Polymers
Polymer flocculants are used extensively in filtration, sedimentation and solids/liquid separation in industries such a water and effluent treatment, and mining. These chemicals form long chain molecular structures with either positive or negative charges. The effect of these chains is to attach to the particles which are to be separated, and cause them to agglomerate into what are generally called floccs, and the process, flocculation.
Flocculation speeds up filtration or sedimentation, and many liquid/solid separation machines are totally reliant on them. Depending on the charge type, they are called either cationic or anionic, and some, with very little charge, are called non- ionic. Polymers are available in different forms, generally powders or concentrated liquid emulsions.
In general, the mining industry tends to use anionic types, and the wastewater and organic industries use mostly cationic types. Some instances, such as clays, require both types of polymer to be applied in order to achieve the required result.
In addition, polymer flocculants are not standard chemicals, and vary greatly, including molecular weight, charge, and polymer chain configuration.
The actual configuration of any particular polymer is a closely guarded trade secret, and manufacturers have even been known to admit that the same polymer, made in two different factories, can produce different performance.
Whilst the theory of flocculation is well known, in practice experience, and trial and error, form the main selection process. A poor quality, or incorrectly selected flocculant will result in reduced process performance.
Polymer flocculant quality
The procurement of polymer flocculants faces a number of difficulties. Firstly, the flocculation characteristics of a particular sludge, be it slurry or biosolids, vary greatly from site to site, and sometimes also from day to day. This means that specifying performance such as drainage rate, flocc strength, or dewatering machine performance, has limited validity.
The other main problem is, as noted above, polymer flocculants are secret formulations, and the customer has no way of accurately assessing what is being supplied.
There is a need to apply practical experience and art to specific applications, and these skills are not always possessed by plant operators. In the past polymer has been sold by teams of skilled specialists, who provided their expertise as part of the cost of the product.
Premature Flocculating Polymers
A new challenge has emerged in recent years. Procurement organisations, without understanding the unique and secret nature of polymer flocculants, and incorrectly viewing them as commodities, have devised large scale contracts for the supply of bulk polymers over extended periods.
Bulk long-term contracts result in minimal technical support, and potentially much more difficulty for the plant operator when faced with changing process conditions or upsets.
This approach has removed the skilled service element and driven the price per kilogram of powder polymers down, but reduced the quality. The bulk buying has led to the emergence in the market place of powder polymers which exhibit what we call “Premature Flocculation”.
Fig 2 shows this effect. A “classical” high quality polymer flocculant , creates floccs starting at a typical one millimetre size, and slowly building in size and strength with additional dosage, until eventually all the particles are captured into large, strong floccs, typically around 30mm diameter.
In contrast, a premature flocculating polymer builds a medium to small flocc size with a lower dosage rate than the classical type, but does not build flocc size and strength with increasing dosage. Many examples also have the characteristic that high dosage can destroy the floccs.
A premature flocculating polymer can sometimes be sufficiently strong to provide the required throughput of the dewatering or filtration equipment, but usually at the expense of the capture rate. (A high percentage of small floccs are lost into the filtrate instead of being captured.) The extended flat part of the dosage curve makes it difficult to control to the optimum dosage setting, and tends in practice to result in over-dosing, which negates any cost saving from the lower cost/weight.
In contrast, liquid polymers tend to have in general, retained “classical” flocculation behaviour, where flocc size and strength increase with dosage, up to maximum which is much greater than the “premature” type. In addition, some modern liquid polymers have minimal increase in viscosity at higher dosage levels. (Less viscosity, faster filtration.) These characteristics have in recent years made liquid polymers the preferred type for high performance and high dewatering shear applications.
In House Knowledge, Skills and testing
Given the variability of polymer flocculant and sludge characteristics the best approach to ensure optimum operation at minimum cost is for customer organizations to have in-house knowledge and skills. Analysis of flocculation characteristics relies heavily on observation and experience, which is best achieved by hands-on operators with daily exposure to the process.
Testing
The basic test for flocculation is a simple bench test, involving drainage time and flocc strength. In both cases, a large strong flocc provides the best result.
A simple drainage test involves flocculating a sample, and then comparing time to drain a fixed percentage of the sample volume through a filter cloth of the same type as the filtration machinery. This test also gives an indication of the sedimentation rate, and hence would also be useful for centrifuge type equipment. On filtration equipment, drainage time is related to the volumetric capacity of the equipment. A belt press which has an efficient large drainage area may tolerate a slower drainage rate than a filter with a small drainage area. Similarly, for a thicker sludge solids content, simple drainage may not be a critical aspect.
Flocc strength is more difficult to measure in a simple bench test. The most common method is to measure the number of beaker pours required to break down the floccs to a small size. A beaker pour is simply pouring from one beaker to another, and is somewhat subjective, but this test is useful for comparative testing.
Other information which can be obtained from bench testing is the amount of mixing required to create the flocc. Fragile floccs usually necessitate very gentle mixing to avoid flocc destruction.
Having established a short list of suitable polymers, these can then be trialed at the full scale. This is still time consuming, as each polymer will have its own optimum mixing and dosing regime, and the variables need to be explored by trial and error.
A hands-on knowledge of the flocculation characteristics gives the operator the best knowledge of the cost/benefit of different polymers. Hence, empowerment of the plant operators in selecting the polymer to be used on their plant, is the most economical methods of procurement, rather than bulk-buying secret formula chemicals.
Equipment Selection
Sludge dewatering and thickening equipment is often specified based on process performance. The dewatering properties of biosolids sludges can vary greatly, and with the added variable of polymer quality, it becomes almost impossible for a dewatering equipment supplier to accurately predict performance without the benefit of a full-scale pilot trial, including extensive polymer trials. This work involves high costs which could amount to a significant proportion of the capital cost of the equipment. Similarly, with high risk for process guarantees, and small profit margins, the supply of dewatering equipment becomes less attractive for reputable suppliers.
Size and Experience is the Key
For many years the recommendation has been to evaluate sludge dewatering and thickening equipment based on the filtration area, and the previous experience of the supplier. Turndown is seldom a problem, and the capital cost of larger dewatering equipment is small compared to both the overall cost of a dewatering system, and the operating cost.
Fig 3 GT 2000 Optima Gravity Drainage Decks
Fig 2 Comparison between conventional and premature flocculating polymers.
Fig 1 Flocculated sludge surrounded by clear water.
Fig 4 Maroochydore QLD, Thickening WAS to 6% solids with liquid emulsion polymer.
